1. Field of the Invention
This invention relates to a quantizer designed by using human visual sensitivity and, more particularly, which permits visually optimum quantization by taking postprocessing after inverse quantization into considerations.
2. Description of the Prior Art
The initial stage quantizer design was frequently made in the light of such a reference as to minimize the mean square of the quantized noise. However, it is the man that visually observes video information. Thus, an adaptive quantization method has been developed, in which coding is made according to the difference of the susceptibility of the human visual sensitivity with the frequency such that portions which are visually important are coded with fidelity while portions which are not important are coded with a certain permissible deterioration. Japanese Laid Open Patent Publication No. 10464/1991 shows techniques concerning this adaptive quantization method.
What expresses the degree of visual importance that varied with the frequency, is the human visual frequency sensitivity, i.e., the visual sensitivity function W(.omega.) (.omega. being the angular frequency). Generally, the visual sensitivity function W(.omega.) is obtained by producing a mathematical model of evaluations made by a plurality of persons as a result of an experiment, in which picture quality deterioration due to noise superimposed for each frequency on a predetermined prepared picture is positively evaluated. Prior art quantizers, which adopts transform coding or like coding in a frequency area, have been designed such as to minimize weighted noise energy which is obtained by weighting the mean square of noise for coding for each frequency component with the visual sensitivity function W(.omega.) and summing the weighted mean squares.
In the visual sensitivity function W(.omega.), generally importance is attached to low frequency components, and high frequency components are suppressed.
The visual sensitivity is a function about picture which is observed by the man, and where some postprocessing is done about the frequency after the quantization, the visual sensitivity can not be used directly for the quantizer design.
For example, in a system as shown in FIG. 6, in which picture information 20 is quantized with a quantizer 21 using the quantization method noted above and then inversely quantized by an inverse quantizer 22 to obtain reproduced picture information 23, by adopting the visual sensitivity function W(.omega.) for the quantizer 21 it is possible to obtain reproduced picture information 23 having satisfactory quality. However, where one postprocessing, for instance with a high-pass filter, is done after the inverse quantization 22 as shown in FIG. 7, the low frequency components of the visual sensitivity function W(.omega.), to which importance is attached, are suppressed in the high-pass filter processing so that they no longer have any importance. Therefore, in the system as shown in FIG. 7 satisfactory quality reproduced picture information 25 can not be obtained by using the visual sensitivity function W(.omega.) for the quantization 21. In this specification, both the processes, i.e., the inverse quantization and postprocessing, are referred to as decoding process.